Apparatus and method for treating a fibrous material web in a long nip press unit

ABSTRACT

The invention relates to an apparatus and a method for treating a fibrous material web ( 9 ) in a paper or cardboard machine in a long nip press unit ( 29 ) comprising a press roll ( 24 ), which has a rotating press jacket ( 25 ) and a mating roll ( 23 ), wherein the fibrous material web ( 9 ) is dewatered in an extended press nip ( 26 ) between the mating roll ( 23 ) and the press jacket ( 25 ) of the press roll ( 24 ). According to the invention, the fibrous material web ( 9 ) is guided after the extended press nip ( 26 ) on a special rotating press jacket ( 25 ) of the press roll ( 24 ) to a transfer region ( 30 ), in which the fibrous material web ( 9 ) is transferred from the press jacket ( 25 ) to a transfer element ( 31, 27 ).

The invention relates to a device for treating a pulp web in a paper or board machine in a long-nip pressing unit with one press roll that has a revolving or rotating press belt and one opposing roll, where the pulp web is dewatered in an extended press nip between the opposing roll and the press belt of the press roll. The invention also refers to a process for treating a pulp web with a device according to the invention.

In conventional paper and tissue production processes, mechanical dewatering of a pulp web before thermal drying takes place by direct pressing of the pulp web onto a dryer (Yankee dryer). A production process of this kind is described in DE 102 33 920 A1. In these paper and tissue machines, however, mechanical pressing or the line load that can be achieved is limited by the pressure because this pressure is applied to the Yankee dryer. By means of a preceding pressing stage, as described in EP 1 075 567 B1 for example, mechanical dewatering is performed in a press unit that is independent of the Yankee dryer. Here, it is possible to set optimum pressing conditions because the pressure is no longer applied to the dryer and thus is not limited by the load limits of the Yankee dryer. Mechanical dewatering can be improved substantially by this preceding pressing stage, which is preferably carried out in a long-nip pressing unit, particularly a shoe press. The effort involved for thermal drying is reduced, thus leading to energy savings.

EP 1 397 553 B1 describes a process to produce a pulp web, where the pulp web is dewatered by means of a shoe press before being transferred to a through-air drum (TAD) for thermal drying.

In conventional processes for producing board, it is state of the art to use one or several separate pressing stages for mechanical dewatering before thermal drying. A press arrangement of this kind is described, for example, in EP 0 954 634 B1. Here, too, the pressing steps are often carried out with one or more long-nip pressing units, such as shoe presses.

In order to be dewatered in a long-nip pressing unit, such as a shoe press, the pulp web is carried on a felt through an extended press nip, which is formed by a shoe press roll with a rotating press belt and an opposing roll. After the extended press nip, the felt is separated from the pulp web as rapidly as possible. The pulp web then continues on a fabric that has also been guided through the extended press nip or is carried onward by the opposing roll.

The press belt of long-nip pressing units has direct contact with a fabric in the extended press nip in conventional plants and is used solely for mechanical dewatering. As a result, the press belts in conventional plants are only optimized for pressing of pulp webs, but not for onward transport of the pulp web. Conventional shoe press belts are not suitable for reliable onward transport of the pulp web.

Onward transport of the pulp web is always performed by other structural elements.

The aim of the present invention is to disclose a process for treating a pulp web, where dewatering and onward transport of the pulp web take place without any web transfer in between the two stages.

In the process according to the invention, the pulp web is guided through an extended press nip of a long-nip pressing unit, where the long-nip pressing unit has a press roll with revolving press belt and an opposing roll. After the extended press nip, the pulp web is carried on the press belt of the shoe press roll to a transfer element to which the pulp web is transferred. It is an advantage if the press roll is a shoe press roll.

The rotating press belt thus not only fulfils its function as a pressing element, but serves at the same time as a means of transporting the pulp web onwards after mechanical dewatering. Thus, there is no need for a transfer fabric that runs through the extended press nip together with the felt in some embodiments.

The invention also seeks to disclose a revolving press belt to achieve reliable onward transport of the pulp web after the press nip.

The revolving press belt according to the invention comprises a supporting material and an elastic, compressible polymer layer. The compressible polymer layer is on the side of the press belt that touches the pulp web. The hardness of the polymer layer is between 50 and 97 Shore A. The surface of the polymer layer, which touches the pulp web, has reversible, pressure-dependent roughness, where this roughness has a value of R_(z)=2-80 microns when pressure-relieved and R_(z)=0-20 microns when pressure-loaded with a line load of 20-600 kN/m. Here, the roughness is measured in accordance with ISO 4287, Part 1. The R_(z) value is the ten-point peak, which is defined in the ISO standard as the average spacing between the five highest peaks and the five lowest valleys of the reference length that is measured at a line running parallel to the centre line and which does not cut through the surface profile.

The pulp web adheres very well to the flexible press belt disclosed as a result of the pressing effect.

The state of the art currently only knows transport belts with the properties mentioned, for example in EP 1 088 131 A1, but not press belts.

It is preferable if the revolving press belt is essentially impermeable, i.e. it has a permeability of less than 6 m³/m²/min, where the permeability is measured according to the “Standard Test Method for Air Permeability of Textile Fabrics, ASTM D 737-75, American Society of Testing and Materials”.

Advantageously the polymer layer can be a polymer compound, such as acrylic polymer resin, polyurethane polymer resin, or a compound of polyurethane and polycarbonate polymer resin.

The polymer layer can also have a particulate filler with a different hardness to the polymer layer itself, for example china clay, a polymer material, or metal, preferably stainless steel.

The invention also relates to a process for treating a pulp web in a paper or board machine in a long-nip pressing unit with one press roll that has a rotating press belt and one opposing roll, where the pulp web is dewatered in an extended press nip between the opposing roll and the press belt of the press roll. According to the invention the process is characterised by the pulp web being carried on the rotating press belt of the press roll after the extended press nip to a transfer zone in which the pulp web is transferred from the press belt to a transfer element, where a press belt with the properties described in Claims 1 to 5 is used.

In the following, the invention is described with the help of drawings, where

FIG. 1 shows a state-of-the-art paper machine with shoe press technology for production of tissue paper,

FIG. 2 shows a state-of-the-art press section of a board machine,

FIG. 3 shows a tissue machine with a press arrangement according to the invention,

FIG. 4 shows a detailed view of the press arrangement according to the invention,

FIG. 5 shows a board machine with the press arrangement according to the invention, and

FIG. 6 shows the progression of a conventional and of a long-nip press profile.

Identical reference numerals in the individual figures refer to identical components.

FIG. 1 shows a conventional tissue machine with shoe press technology. The pulp suspension is fed to the forming unit through a headbox 1 and exits from the headbox 1 between a breast roll 4 and a forming roll 5. An outer fabric 2 is wrapped round the breast roll 4. In the forming unit, the pulp suspension is dewatered far enough for a pulp web 9 to form on the fabric 3. The fabric 3 is preferably a felt that carries the pulp web 9 to a shoe press roll 6. An extended press nip in which the pulp web is dewatered mechanically and transferred to the Yankee dryer 7 is formed between the shoe press roll 6 and the Yankee dryer 7. Thermal drying of the pulp web 9 takes place on the Yankee dryer 7. A doctor 8 detaches the dry pulp web 9 from the Yankee dryer 7. Due to direct pressing of the pulp web 9 onto the Yankee dryer 7, mechanical dewatering is limited because the shoe press roll 6 cannot be pressed onto the Yankee dryer 7 at any desired force for reasons of stability. The maximum line load is generally limited to 170 kN/m.

As a result of the press arrangement according to the invention, mechanical dewatering of the pulp web 9 can be increased substantially compared with the press arrangement in FIG. 1.

FIG. 2 shows a schematic view of a press section 11 of a state-of-the-art board machine. The press section 11 here is arranged after a wet section 10 and before a dryer section 12. The pulp web 9 is transferred from the wet section 10 to the press section 11 by the wire 13. Web transfer to the press felt 14 a is assisted by the transfer roll 15, to which suction is applied. In the press section 11, the pulp web 9 is dewatered mechanically by the two shoe presses 16 a and 16 b. The shoe presses 16 a, 16 b each consist of a shoe press roll 18 a, 18 b and an opposing roll 17 a, 17 b. An extended press nip in which the pulp web 9 is dewatered mechanically is formed respectively between the shoe press rolls 18 a, 18 b and the opposing rolls 17 a, 17 b. The moisture from the pulp web 9 is absorbed during this process by the press felts 14 a, 14 b, 14 c and 14 d, which are also guided through the extended press nip together with the pulp web 9.

After mechanical dewatering, the pulp web 9 is transferred to the dryer fabric 21 in the dryer section 12 with the aid of the transfer roll 22. In the dryer section 12, the pulp web 9 is carried on the dryer fabric 21 in a meandering path over the dryer 19 and the suction rolls 20, undergoing thermal drying at the same time.

FIG. 3 now shows a tissue machine with a press arrangement according to the invention. It consists of a long-nip pressing unit 29, containing a press roll 24 with rotating press belt 25 and an opposing roll 23. An extended press nip 26 is formed between the press roll 24 and the opposing roll 23.

The press arrangement according to the invention now operates as follows:

The pulp web 9 is carried through the extended press nip 26 on the felt 33. In the extended press nip 26, the felt 33 absorbs moisture from the pulp web 9. In the present example, the felt 33 has a three-dimensional structure. The pulp web 9 can therefore give way into the three-dimensional structure of the felt 33 during the pressing process. Thus, pressure is applied to specific points and not over an area. The felt 33 is separated from the pulp web 9 directly after the extended press nip 26 so that re-wetting is avoided.

After the extended press nip 26, the pulp web 9 no longer runs on the felt 33, but on the press belt 25. The press belt 25 comprises a supporting material and an elastic, compressible polymer layer. The compressible polymer layer is on the side of the press belt in contact with the pulp web. The hardness of the polymer layer is between 50 and 97 Shore A. The surface of the polymer layer, which touches the pulp web, has reversible, pressure-dependent roughness, where this roughness has a value of R_(z)=2-80 microns when pressure-relieved and R_(z)=0-20 microns when pressure-loaded with a line load of 20-600 kN/m. The press belt 25 is essentially impermeable, i.e. it has a permeability of less than 6 m³/m²/min.

The supporting material can have a multi-layer woven structure, for example, made of a polymer monofilament yarn such as polyester, polyamide and similar.

In a transfer zone 30, the press belt 25 passes the pulp web 9 on to a transfer element 31. In the present example, the transfer element 31 is a transfer fabric 27. On the other hand, the transfer element 31 can also be a roll that receives the pulp web 9 from the press belt 25. Suction can also be applied to this roll.

Transfer of the pulp web to the transfer fabric 27 is assisted by the suction roll 28. An extended transfer nip 32 is formed between the suction roll 28 and the pressing roll 24.

In the present example, the transfer fabric 27 is permeable, but it is of course quite conceivable to use a non-permeable transfer fabric 27. The transfer fabric 27 can have either a smooth or a structured surface.

A further processing stage for the pulp web 9, namely either creping or stretching of the pulp web 9, can be carried out in the transfer zone 30. For creping, the surface of the transfer fabric 27 moves a little more slowly (lower relative speed) through the extended transfer gap 32 than the press belt 25, thus causing the pulp web 9 to be compressed or creped when it is passed on to the transfer fabric 27. Conversely, it is also possible to apply tensile forces to the pulp web 9 that result in the pulp web 9 being stretched. In order to achieve this, the transfer fabric 27 moves a little faster (higher relative speed) than the press belt. A rapidly moving transfer fabric 27 can have a positive effect on transfer of the pulp web.

The transfer fabric 27 should be conditioned in such a way that there is no or only very little wetting of the transfer fabric 27 as a result of the conditioning process. Thus, conditioning can be performed with compressed air, for example, or a compressed air lance. If water is used for conditioning, it must be guaranteed that the transfer fabric 27 is dried or dried by suction before the pulp web 9 is transferred to it again.

In order to stabilize the pulp web 9 on the transfer fabric 27, it can be advantageous if suction is applied continuously to the zone in which the transfer fabric 27 carries the pulp web 9. After mechanical dewatering in the long-nip pressing unit 29, the pulp web 9 undergoes thermal drying on a Yankee dryer 7. The dry pulp web 9 is scraped off the Yankee dryer 7 with the aid of a doctor 8.

FIG. 3 also shows the adjustable felt roll 40. This adjustable felt roll 40 can be used to change the exit angle of the felt 33 from the long-nip pressing unit 29, for example by +/−15°. In this way it is possible to alter the contact area or the contact length of the felt 33 with the pulp web 9 after the extended press nip 26. The adjusting function of the felt roll 40 is indicated by a double arrow.

By enlarging this contact area or increasing this contact length, it is possible to ensure that the felt 33 runs on the press belt 25 for a little longer, with the pulp web 9 being clamped between the felt 33 and the press belt 25. This is beneficial to the pulp web 9 running on the press belt 25. Any reduction in this contact area between the felt 33 and the pulp web 9 has the effect of enabling the felt 33 to be separated from the pulp web 9 particularly quickly after the extended pressing gap 26. In order to transfer a web tail strip, the largest possible contact area should be set at first so that the transfer tail runs securely on the press belt 25. When the pulp web 9 has been transferred and broadened to its full width, this contact area can be reduced again so that re-wetting of the pulp web 9 is kept to a minimum.

The pulp web 9 can be heated by means of the steam blow box 36.

The long-nip pressing unit 29 is illustrated in more detail in FIG. 4. Here, the extended press nip 26 is shown clearly between the press belt 25 of the press roll 24 and the opposing roll 23. Similarly, the extended transfer gap 32 between the press belt 25 of the press roll 24 and the transfer fabric 27 is clearly visible. The press roll 24 is designed as a shoe press roll.

The extended transfer gap 32 is formed by the transfer fabric 27 being pressed against the press belt 25 by a roll, in the present case a roll to which suction is applied 28, where the press belt 25 largely follows the surface contour of the roll 28 in the transfer zone 30. The supporting and guide surface of the press roll 24 for the press belt 25 is formed in such a way in the transfer zone 30 that the press belt 25 is pressed in towards the central axis 39 of the press roll 24 in this zone, similar to the way in which this is effected in the area of the extended press nip 26. By changing the press-down depth of the press belt 25, the length of the extended transfer gap 32 can be modified.

Overpressure is applied to the inside of the press roll 24 and serves to stabilise the rotating press belt 25. The face ends of the pressing roll 24 have suitable sealing end covers.

The opposing roll 23 of the long-nip pressing unit 29 can have grooves across the machine running direction in order to enhance dewatering. In this case, the grooves should be as narrow and as close to one another as possible as this can improve dewatering considerably. A groove width of less than 0.5 mm, particularly 0.4 mm, and a groove number of 5 or more per centimetre, viewed in the circumferential direction of the opposing roll 23, is desirable. The surface jacket of the opposing roll 23 can be made of a hard elastomer or of metal; grooves can be cut into these materials very well.

In FIG. 4, cleaning devices 34 are provided for cleaning the press belt 25 after the transfer zone 30. The cleaning devices 34 can comprise one or several doctors, but may also include spray nozzles for a cleaning fluid, such as water or air. The cleaning device 34 can also be used for lifting off the web when transferring the pulp web 9. The pulp web 9 can thus be lifted off the press belt 25, scraped off for example, and fed to a pulper until web running has stabilized and the pulp web 9 can be fed to the dryer section.

In addition, a boundary surface adhesion mixture can be applied to the surface of the press belt 25 before the press belt 25 passes through the extended press nip 26. It can be applied using, for example, a spray bar 35 with showers that spray the boundary surface adhesion mixture onto the press belt 25. The surface adhesion of the pulp web 9 on the press belt 25 can be influenced with this process step.

All fluids that are used for surface treatment of Yankee dryers 7, as well as TAD chemicals, can be used for this purpose.

Dewatering of the pulp web 9 in the extended press nip 26 can also be improved by heating the pulp web 9, with the aid of a steam blow box 36 for example, arranged in front of the extended press nip 26. The adjustable felt roll 40 is also shown.

FIG. 5 shows a board machine according to FIG. 2, however in this case the machine has a long-nip pressing unit 29 according to the invention. Here, the pulp web 9 is transferred directly from the press belt 25 to the dryer fabric 21 in the dryer section 12. Pulp web transfer in the extended transfer nip 32 is assisted by the roll 28, to which suction is applied. The length of the extended transfer nip 32 can be adjusted via the press-down depth into the press roll 25 by the roll 28, to which suction is applied.

A comparison with FIG. 2 shows that the pressing felt 14 c is no longer required in the board machine according to the invention.

In the extended press nip 26, a precisely defined pressing profile can act upon the pulp web 9. This type of pressing profile for a long-nip pressing unit 29 is shown as curve 37 in FIG. 6. Curve 38 shows a pressing profile of a pressing unit with standard rolls without an extended press nip 26. The adjustable felt roll 40 allows the contact area or contact length between felt 33 and pulp web 9 after the extended press nip 26 to be set here as well.

A steam blow box (not shown) can also be provided here in order to heat the pulp web 9 ahead of the extended press nip 26.

In the long-nip pressing unit 29, the pressing force applied to the pulp web 9 when it enters the extended press nip 26 should preferably be as low as possible. This pressing force then increases slowly, as shown clearly by curve 37 in FIG. 6. As a result of the gentle rise in pressing force, the specific volume (bulk) of the pulp web 9 is retained. As the dryness of the pulp web 9 rises, the pressing force for further dewatering can also be increased without having any substantial impact on the bulk. At a dry content of 40 to 50%, the pressing force reaches a maximum. At the end of the extended press nip 26, the pressing force should drop again as rapidly as possible as this will largely prevent or minimize the pulp web 9 being re-wetted by the felt 33.

The embodiments shown in the drawings merely illustrate a preferred embodiment of the invention. The invention also includes other embodiments, where the transfer fabric 27 is wrapped partly round the press belt 25 in the transfer zone 30 for example. This also leads to formation of an extended transfer nip 32 for transfer of the pulp web 9. 

1. A press arrangement of a paper or board machine for treating a pulp web (9), in which the pulp web (9) is guided through an extended press nip (26) of a long-nip pressing unit (29), the long-nip pressing unit (29) has a press roll (24) in the form of a shoe press with a revolving press belt (25) and an opposing roll (23), the pulp web (9) is carried on the press belt (25) of the press roll (24) after the extended press nip (26) to a transfer element (31, 27) to which the pulp web (9) is transferred, wherein the improvement comprises that the pulp web (9) is carried on a felt (33) to and through the extended press nip (26), the revolving press belt (25) includes a supporting material and an elastic, compressible polymer layer, the polymer layer has a surface that faces the pulp web and has a hardness between 50 and 97 Shore A, and said surface of the polymer layer contacts the pulp web and has reversible, pressure-dependent roughness of R_(z)=2-80 microns, measured in accordance with ISO 4287, when pressure-relieved and R_(z)=0-20 microns when pressure-loaded with a line load of 20-600 kN/m.
 2. The press arrangement according to claim 1, wherein the revolving press belt (25) is essentially impermeable.
 3. The press arrangement according to claim 1, wherein the polymer layer is a polymer compound, selected from the group consisting of acrylic polymer resin, polyurethane polymer resin, and a compound of polyurethane and polycarbonate polymer resin.
 4. The press arrangement according to claim 1, wherein the polymer layer includes a particulate filler.
 5. The press arrangement according to claim 1, wherein the polymer layer completely encloses the supporting material.
 6. (canceled)
 7. The press arrangement according to claim 2, wherein the polymer layer is a polymer compound, selected from the group consisting of acrylic polymer resin, polyurethane polymer resin, and a compound of polyurethane and polycarbonate polymer resin.
 8. The press arrangement according to claim 2, wherein the polymer layer includes a particulate filler.
 9. The press arrangement according to claim 2, wherein the polymer layer completely encloses the supporting material.
 10. The press arrangement according to claim 3, wherein the polymer layer includes a particulate filler.
 11. The press arrangement according to claim 4, wherein the filler is selected from the group consisting of china clay, a polymer material, and a metal.
 12. The press arrangement according to claim 8, wherein the polymer layer completely encloses the supporting material.
 13. The press arrangement according to claim 12, wherein the polymer layer is a polymer compound, selected from the group consisting of acrylic polymer resin, polyurethane polymer resin, and a compound of polyurethane and polycarbonate polymer resin; and the filler is selected from the group consisting of china clay, a polymer material, and a metal.
 14. A process for treating a pulp web (9) in a paper or board machine in a long-nip pressing unit (29) with one press roll (24) that is formed as a shoe press roll and which has a rotating press belt (25) and one opposing roll (23), wherein the pulp web (9) is dewatered in an extended press nip (26) between the opposing roll (23) and the press belt (25) of the press roll (24), the pulp web (9) is carried on the rotating press belt (25) of the press roll (24), after the extended press nip (26) to a transfer zone (30) in which the pulp web (9) is transferred from the press belt (25) to a transfer element (31, 27), the pulp web (9) is carried on a felt (33) to and through the extended press nip (26), in contact with the rotating press belt (25) which press belt (25) includes a supporting material and an elastic, compressible polymer layer, the polymer layer has a surface that faces the pulp web and has a hardness between 50 and 97 Shore A, and said surface of the polymer layer contacts the pulp web and has reversible, pressure-dependent roughness of R_(z)=2-80 microns, measured in accordance with ISO 4287, when pressure-relieved and R_(z)=0-20 microns when pressure-loaded with a line load of 20-600 kN/m.
 15. The process of claim 14, wherein the revolving press belt (25) is essentially impermeable
 16. The process of claim 14, wherein the polymer layer is a polymer compound, selected from the group consisting of acrylic polymer resin, polyurethane polymer resin, and a compound of polyurethane and polycarbonate polymer resin.
 17. The process of claim 14, wherein the polymer layer completely encloses the supporting material.
 18. The process of claim 14, wherein the polymer layer completely encloses the supporting material.
 19. The process of claim 15, wherein the polymer layer includes a particulate filler.
 20. The process of claim 19, wherein the polymer layer completely encloses the supporting material. 